The present invention relates generally to power supply circuitry and more particularly, relates to a sequencing circuit for applying a highest voltage source to a chip.
In known electronic systems, various DC voltage levels often are required that may be provided by multiple different DC supplies. For example, one chip may have a voltage power rail Vdd connected to a 5 volt power supply and include components and input/output (I/O) that use a 3.3 volt power supply. Typically, a system power supply does not instantly provide the correct supply voltages during startup or power down of an electronic system, such as a computer system. Known power supplies have a startup delay when the system is powered on and also a bring down delay when the when the system is powered off.
A sequencing problem exists with some chips included in such known electronic systems. For example, one chip has a voltage power rail Vdd connected to the +5 volts of the system, and this chip also used 3.3 volts and had I/O pins that were also pulled up or wired to other chips that also used the 3.3 volt supply. This chip would latch up if at any time the voltage power rail Vdd is less than any other voltage that was used by this chip. This sequencing problem required that the voltage power rail Vdd be maintained as the highest voltage to the chip during power up and power down of the system.
One way that this sequencing problem has been dealt with in the past is to require the power systems to sequence up the +5 volt supply of the system first and then to power up the 3.3 volt supply next. Then the opposite sequence has been required during power down with the 3.3 volt supply powered down first, then the +5 volts of the system is powered down. This required sequencing of multiple power supplies adds complexity and cost to the power systems and this sequencing of the power supplies may result in other problems with other chips in the system.
A second technique for accommodating this sequencing problem uses in line switches, such as field effect transistors (FETs), to switch the 3.3 volt supply on after the +5 volt supply is powered up. Then the in line switches or FETs switch the 3.3 volt supply off before the +5 volt supply is powered down. This method requires that all chips running off the 3.3 volt supply that are coupled to I/O of the system chips also need to be switched, so that the I/O voltages are also controlled. This method would require a very large switch; for example, a switch rated for 20 Amps or more may be required. It also would sequence other chips in the system in a way that may cause other problems.
A principal object of the present invention is to provide a sequencing circuit for applying a highest one of system voltage supplies to a chip. Other important objects of the present invention are to provide such sequencing circuit for applying a highest one of system voltage supplies to a chip substantially without negative effect and that overcome many of the disadvantages of prior art arrangements.
In brief, a sequencing circuit and sequencing method are provided for applying a highest applying a highest voltage of first and second system supplies to a chip. The sequencing circuit includes a first transistor coupled between the first system power supply and a power supply input to the chip and a second transistor coupled between the second system power supply and a power supply input to the chip. The sequencing circuit includes a comparator for sensing a highest voltage of the first and second system power supplies. The first transistor and second transistor are coupled to an output of the comparator. When the comparator senses that the first power supply voltage is higher than the second power supply voltage, then the first transistor is turned on and couples the first power supply voltage to the power supply input to the chip. Alternately, when the comparator senses that the second power supply voltage is higher than the first power supply voltage, then the second transistor is turned on and couples the second power supply voltage to the power supply input to the chip.